I'll try to limit myself to actual arguments instead of ad hominem remarks.

-> Different languages: Maybe for english speakers it's difficult to grasp, but there are really many different languages. In Belgium there are 3 official languages, and if you have something remotely to do with Internet you also use English. So which language would you set in mIRC?

-> Dialects: On IRC, dialects, regional or spoken language are used a lot. In Dutch there is a difference between written language and IRC language. Yes it's not correct, but it is widely accepted. There are still errors (typo's and incorrect grammar for some things), but some errors are intentional for a more fluent or natural message.
I have never seen a spell checker that works perfectly with this language, I might even look at it for 2 seconds...

-> "This may not be true, when they see a mispelled word, chances are they'll try and correct it"
No, chances are they'll get annoyed by that wavy red line stuff and get a client that doesn't act like a school teacher.

-> Spell checkers suck: they often give wrong output, both overlooking errors and marking errors where there are none. Same for proper names, nick names and stuff like that. I'm not going to add hundred different nicknames to have a rather clean looking mIRC.

-> mIRC is a chat program; not a spelling teacher. (If you want it to be both, go to next point)

And now the important stuff:
-> You can script it. Use com objects or code a dll and have it check the text with on INPUT events, or make your own dialogs or whatever. You apparently already have all the dictionaries you need... You can even write on @*:TEXT*:#:if ($dll(check.dll,check,$1-)) kick # $nick My eyes hurt because of you. Do note that I won't be rejoining your channel.